Passenger cars or trucks which are operated with internal combustion engines have become a permanent part of modern society. The automobile industry is aiming to develop vehicles which are distinguished by low emissions of pollutants and at the same time can be manufactured in a cost-effective manner. In particular, at present, development work is centred on nitrogen oxide reduction technologies. Therefore, new methods for exhaust gas purification are being developed in order to reduce the concentration of nitrogen oxide (NOx) in exhaust gases. One method is to use an ammonia SCR system. This system is used, in particular, for lowering the NOx emissions of both trucks and passenger cars.
In some SCR systems, a urea solution is injected as a reducing agent into the exhaust system of the vehicle. This liquid reducing agent is vaporized in the exhaust system and is ultimately converted into gaseous ammonia (NH3). With the aid of this ammonia, the noxious nitrogen oxides NOx are converted into nitrogen N2 and water H2O in the ammonia SCR catalytic converter. So that the ammonia SCR reaction can take place, ammonia must be firstly adsorbed, that is to say stored in the SCR catalytic converter. The conversion of NOx can depend to a great extent on the quantity of stored ammonia, in particular at low catalyst temperatures.